Can you spot the bug in the following code?, options.url, options.sync);

The problem is that the last param to is whether the xhr is async. Ouch.

Turns out, this code was written intentionally, and used browser sniffing so it only failed on Gecko. It appears that the author believed that Gecko's sync/async argument went in the opposite direction as WebKit's (maybe this was true at some point in the past?). This has been fixed as of a few hours ago, but it seems that the code was broken on Gecko for two years.

As further illustration, what does the boolean parameter here mean?

nsCOMPtr observer = ...;
mDocument->AddObserver(observer, "load", true);

Even if you're familiar with this interface, and you know that the first boolean parameter has to do with strong vs. weak references, do you remember whether true means strong or weak? I never can, because there's no obvious way to map from true/false to strong/weak -- it's arbitrary, just as the true/fase to sync/async mapping is arbitrary.

(In case it's not clear, boolean parameters are perfectly fine when the parameter really is a boolean. For example, nobody will be confused by setVisiblity(false);.)

We can do better than this.

When you have an API which takes a boolean parameter which chooses between two behaviors, just make two API functions, and put the behavior in the function name! See how much clearer these APIs become if we do this?

xhr.openAsync(options.type, options.url)
mDocument.AddWeakObserver(observer, "load");

We should demand clear APIs, both from ourselves and, where we can, from others. Functions are cheap, but bugs are not.